1. Field of the Invention
The present invention relates to an information recording medium on/from which information can be recorded, erased, rewritten and reproduced optically or electrically and a method for producing the same.
2. Description of the Related Art
An example of the basic structure of an optical information recording medium is such that a first dielectric layer, a recording layer, a second dielectric layer and a reflective layer are formed in this order on a surface of a substrate. The first and the second dielectric layers serve to regulate the optical distance to increase the optical absorption efficiency of the recording layer and increase the difference between the reflectance of the crystalline phase and the reflectance of the amorphous phase to increase the signal amplitude. The dielectric layers also serve to protect the recording layer from moisture or the like. The dielectric layers are formed of, for example, a mixture of 80 mol % of ZnS and 20 mol % of SiO2 (hereinafter, expressed by of “(ZnS)80 (SiO2)20 (mol %)” or “(ZnS)80 (SiO2)20”; other mixtures also are expressed in the same manner) (e.g., see Japanese Patent 1959977). This material is amorphous and has a low thermal conductivity, high transparency and a high refractive index. This material also has a high film-formation speed when a film is being formed, and excellent mechanical characteristics and moisture resistance. Thus, “(ZnS)80 (SiO2)20” has been put into practical use as a material suitable for forming dielectric layers.
With a recent tendency toward increasing high density, the recording layer has been designed to be as thin as about ⅓ of that in 1990, the year that is in the early period of the practical use. This is for the purpose of reducing the heat capacity of the recording layer and letting heat escape to the reflective layer side rapidly after the temperature is increased in order to record small marks satisfactorily.
The inventors of the present invention found out a problem of (ZnS)80(SiO2)20 as the thickness of the recording layer is reduced. The problem is a phenomenon where, when the recording layer is irradiated with laser light to rewrite information repeatedly, S in (ZnS)80(SiO2)20 is diffused in the recording layer and the repeated rewriting performance is reduced significantly. In order to prevent this diffusion, the inventors of the present invention proposed that layers for serving as interface layers should be provided between the first dielectric layer and the recording layer and between the recording layer and the second dielectric layer (e.g., see N. Yamada et al., Japanese Journal of Applied Physics Vol. 37(1998) pp. 2104-2110). A nitride containing Ge is disclosed as the material of the interface layers (e.g., see WO 97/34298). Materials containing S are not suitable. The interface layers improved the repeated rewriting performance significantly. An interface layer is provided in a 4.7 GB/DVD-RAM (Digital Versatile Disk-Random Access Memory) disk that already has been in practical use, such as an information recording medium 31 shown in FIG. 12. In this medium, a first dielectric layer 102, a first interface layer 103, a recording layer 4, a second interface layer 105, a second dielectric layer 106, an optical absorption correcting layer 7 and a reflective layer 8 are formed in this order on a surface of a substrate 1, and a dummy substrate 10 is attached to the reflective layer 8 with an adhesive layer 9 (e.g., see JP2001-322357). This configuration can provide large capacity and excellent repeated rewriting performance.
A layer made of a nitride containing Ge can be formed by using Ge or an alloy containing Ge for reactive film-formation in a high pressure atmosphere with a mixture of Ar gas and nitrogen gas. The repeated rewriting performance or the moisture resistance depend on the degree of this nitriding of Ge, so that the conditions for film-formation are determined strictly. In particular, reactive film-formation at a high pressure depends significantly on the structure of a film-formation apparatus and the conditions for film-formation. For example, it took time to determine the conditions for optimal pressure or gas flow rate when a film-formation apparatus for experiments was scaled up to a film-formation apparatus for mass production. Since there is such a problem, there is a demand for a material with which an interface layer can be formed by non-reactive film-formation, that is, can be formed in a low pressure atmosphere of Ar gas and is free from S. If the interface layer is used as a dielectric layer, it is possible to reduce the number of layers.
Furthermore, an example of materials suitable for the interface layer of an information recording medium is one proposed from the viewpoint of the relationship of the thermal conductivity (e.g., see JP2001-67722).
In order to solve the above-described conventional problems, the inventors of the present invention have proposed an interface layer that can be formed by non-reactive film-formation and has excellent moisture resistance and repeated rewriting performance, that is a dielectric material that can be provided in contact with the recording layer, can be used as the first or the second dielectric layer, and contains a mixture of ZrO2, SiO2 and Cr2O3, which exhibit excellent repeated rewriting performance. In this ZrO2—SiO2—Cr2O3, ZrO2 and SiO2 are transparent and thermally stable materials, and Cr2O3 is a material that has excellent adhesion with a chalcogen based recording layer. Therefore, both the thermal stability and the adhesion can be provided by mixing these three oxides. In order to ensure adhesion, a composition containing Cr2O3 in a content of 30 mol % or more is more preferable. An information recording medium in which ZrO2—SiO2—Cr2O3 material is used as an interface layer or a dielectric layer has excellent repeated rewriting performance and moisture resistance.
However, low thermal conductivity and transparency also are required for a material suitable for forming an interface layer or a dielectric layer. These two properties of ZrO2—SiO2—Cr2O3 are not comparable to those of (ZnS)80 (SiO2)20. ZrO2 and SiO2 are substantially optically transparent (extinction coefficient 0.00 or less) in wavelength regions of 660 nm and 405 nm, whereas Cr2O3 absorbs light in the two regions and is not transparent. Cr2O3 absorbs light in a larger amount as the wavelength becomes shorter, and the extinction coefficient in the vicinity of 405 nm is nearly 0.3. For this reason, for example, in the case of a composition of mixed (ZrO2)25(SiO2)25(Cr2O3)50 (mol %), the extinction coefficient in a wavelength region of 660 nm is 0.02, and the extinction coefficient in a wavelength region of 405 nm is 0.2. If the material is not transparent, the dielectric layer absorbs light, which reduces light absorption of the recording layer and increases the temperature of the dielectric layer. Phase change recording is performed by forming an amorphous mark in the recording layer by melting a laser light irradiation portion and cooling it rapidly (recording) and heating it to the crystallization temperature or more and then cooling it gradually for crystallization (erasure). When the light absorption of the recording layer is reduced, the recording sensitivity and the erasure sensitivity of the recording layer are reduced (laser light irradiation with larger power is necessary). When the temperature of the dielectric layer is increased, this makes it difficult to cool the recording layer rapidly and form satisfactory amorphous marks during recording. As a result, the signal quality is deteriorated.
Regarding the thermal conductivity, since it is difficult to measure the thermal conductivity of a thin film precisely, the magnitudes of the thermal conductivities are compared relatively, based on the difference in recording sensitivity between individual information recording media. For example, when the thermal conductivity of the second dielectric layer is low, heat is accumulated temporarily in the recording layer and then is diffused rapidly to the reflective layer without being diffused in the in-plane direction. In other words, the rapid cooling effect is increased, so that amorphous marks can be formed with a smaller laser power (high recording sensitivity). On the other hand, when the thermal conductivity of the second dielectric layer is high, heat is hardly accumulated in the recording layer and easily is diffused to the second dielectric layer. Thus, the rapid cooling effect is small and a large laser power is necessary to form amorphous marks (low recording sensitivity). For ZrO2—SiO2—Cr2O3, a larger laser power is required for recording than when (ZnS)50(SiO2)20 is used as the second dielectric layer, so that it is determined that for ZrO2—SiO2—Cr2O3 has high thermal conductivity.
Thus, ZrO2—SiO2—Cr2O3 has problems in thermal conductivity and transparency. When ZrO2—SiO2—Cr2O3 is used as the first or the second dielectric layer in DVD-RAM disks and Blu-ray disks, the recording sensitivity is low, which requires improvement.